To obtain hydrocarbons such as oil and gas, wellbores (also called the boreholes) are drilled by rotating a drill bit attached at the end of a drilling assembly generally called the “bottom hole assembly” or the “drilling assembly.” A large portion of the current drilling activity involves drilling highly deviated or substantially horizontal wellbores to increase the hydrocarbon production and/or to withdraw additional hydrocarbons from the earth's formations. The wellbore path of such wells is carefully planned before drilling such wellbores using seismic maps of the earth's subsurface and well data from previously drilled wellbores in the associated oil fields. Due to the very high cost of drilling such wellbores and the need precisely to place such wellbores in the reservoirs, it is essential continually to determine the position and direction of the drilling assembly and thus the drill bit during drilling of the wellbores. Such information is used, among other things, to monitor and adjust the drilling direction of the wellbores.
In drilling assemblies used until recently, the directional package commonly includes a set of accelerometers and a set of magnetometers, which respectively measure the earth's gravity and magnetic field. The drilling assembly is held stationary during the taking of the measurements from the accelerometers and the magnetometers. The toolface and the inclination angle are determined from the accelerometer measurements. The azimuth is then determined from the magnetometer measurements in conjunction with the tool face and inclination angle.
The earth's magnetic field varies from day to day, which causes corresponding changes in the magnetic azimuth. The varying magnetic azimuth compromises the accuracy of the position measurements when magnetometers are used. Additionally, it is not feasible to measure the earth's magnetic field in the presence of ferrous materials, such as casing and drill pipe. Gyroscopes measure the rate of the earth's rotation, which does not change with time nor are the gyroscopes adversely affected by the presence of ferrous materials. Thus, in the presence of ferrous materials the gyroscopic measurements can provide more accurate azimuth measurements than the magnetometer measurements. U.S. Pat. No. 6,347,282 to Estes et al having the same assignee as the present application and the contents of which are fully incorporated herein by reference, discloses a measurement-while-drilling (MWD) downhole assembly for use in drilling boreholes that utilizes gyroscopes, magnetometers and accelerometers for determining the borehole inclination and azimuth during the drilling of the borehole. The downhole assembly includes at least one gyroscope that is rotatably mounted in a tool housing to provide signals relating to the earth's rotation. A device in the tool can rotate the gyroscope and other sensors on the tool at any desired angle. This ability to rotate the sensors is important in determining bias in the sensors and eliminating the effects of the bias.
U.S. Pat. No. 5,091,644 to Minette having the same assignee as the present application teaches a method for analyzing data from a measurement-while-drilling (MWD) gamma ray density logging tool which compensates for rotations of the logging tool (along with the rest of the drillstring) during measurement periods. In accordance with the method disclosed therein, the received signal is broken down into a plurality of sections. In a preferred embodiment, the Minette invention calls for the breaking of the signal from the formation into four different sections: top, bottom, right, left. As the tool rotates, it passes through these four quadrants. Each time it passes a boundary, a counter is incremented, pointing to the next quadrant. This allows for dividing the data into four spectra for each detector. Each of these four spectra will be obtained for ¼th of the total acquisition time assuming constant rotational speed.
U.S. Pat. No. 6,307,199 to Edwards et al teaches the use of a density gamma ray logging device in which data from different “azimuthal” sectors are combined to give an interpretation of formation dip. The primary emphasis in both the Minette and Edwards patent is to correct the density measurements for the effects of standoff; the sensors themselves are not specifically designed for “azimuthal” sensitivity. U.S. Pat. No. 6,215,120 to Gadeken et al. discloses the use of “azimuthally” focused gamma ray sensors on a logging tool for detecting “azimuthal” variations in the gamma ray emission from earth formations.
Other types of images have been obtained in the prior art using sensors on a rotating bottom hole assembly (BHA). For example, U.S. Pat. No. 5,200,705 to Clark et al. discusses resistivity measurements made by a galvanic resistivity sensor on a stabilizer blade. U.S. Pat. No. 6,173,793 to Thompson et al., having the same assignee as the present invention and the contents of which are fully incorporated herein by reference, teaches the use of pad mounted sensors on a slowly rotating sleeve for obtaining azimuthal resistivity images of the borehole wall. Resistivity images can, for the purpose of the present invention, be considered to be similar to images obtained with a nuclear logging tool, albeit with a much higher resolution. Resistivity measurements do not suffer from the statistical variability associated with nuclear logging tools and can hence make virtually instantaneous measurements yielding images with a much higher resolution than can nuclear measurements.
We digress briefly on a matter of terminology. In surveying, the term “azimuth” usually refers to an angle in a horizontal plane, usually measured from north: when referenced to magnetic north, it may be called magnetic azimuth and when referenced to true north, it is usually simply termed azimuth. It would be clear based on this definition that all measurements made in a highly deviated borehole or a horizontal borehole would be made with substantially the same azimuth. Accordingly, in the present application, we use the more accurate term “tool face angle” to define a relative orientation in a plane orthogonal to the borehole axis. With this definition, the Minette, Edwards and Gadeken patents are really making measurements over a variety of tool face angles.
Common to the Minette, Edwards and Gadeken patents is the use of a controller that keeps track of the rotating sensor assembly and controls the acquisition of data based on sector boundaries in the tool face angle. While this may not be difficult to do for the case of a single directionally sensitive sensor, the problem becomes much more complicated when a plurality of different types of sensors are conveyed as part of a bottom hole assembly. It is difficult, if not impossible, for a single controller to keep track of a plurality of sensor assemblies during rotation of the downhole assembly and control the operation of a plurality of assemblies. A source of error is the nonuniform rotation speed of the drillstring. Another source of error is the time delay inherent in the electronics. Measurements may be made simultaneously by the formation sensor and the orientation sensors, but there is a time delay between the time the measurements are made with the two types of sensors and the time at which they are processed. The interaction between the two sources of error, i.e., nonuniform rotation and time delay, can be fairly complex. The problem of nonuniform rotation is partially addressed in copending U.S. patent application Ser. No. 10/629,268 of Cairns et.al. having the same assignee and the contents of which are fully incorporated herein by reference. However, addressing the non-uniform rotation by itself gives only a partial solution. In addition, there is the problem of bias in the orientation sensor measurements. Generally, magnetometers are preferred as orientation sensors over gyroscopes, but magnetometers are susceptible to errors causes by metallic drill collars, casing, and accumulated debris. There is a need for a method of determining accurate orientation values using measurements made by a an orientation sensor on a MWD logging tool. It would be desirable to have an apparatus and a method that efficiently controls data acquisition and possibly processing with a plurality of rotating sensors in a downhole device. The present invention satisfies this need.